It is possible to classify the prior art of commercial grain refiners in two main catagories based on chemical composition, and these two catagories can be broken down further into two subcategories based on structure. This classification is depicted below:
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This classification runs counter to that usually found in the prior art. In the past, the primary means used to describe a grain refiner was the bulk chemistry of the alloy. Even the use of the word "alloy" is somewhat questionable. Since the solubilities of titanium and boron in liquid aluminum metal are small, nearly all titanium and boron are present as TiAl.sub.3 and boride crystals. Therefore, changing the bulk composition of the alloy only changes the relative proportion of these three phases: aluminum metal, aluminides, and borides.
The morphology of the aluminide crystals in aluminum-titanium alloys is determined by the process used to produce this material. For a needle-like structure, the titanium mus first be put into liquid solution at high temperature. Then TiAl.sub.3 will precipitate in needle form upon cooling. The size of needles is dependent on the cooling rate. The blocky structure results from a growth of TiAl.sub.3 directly from the source of titanium in the presence of a liquid solution saturated in titanium. This occurs at temperatures where the solubility of titanium in the liquid is fairly small; i.e., less than about 900.degree. C. The blocky crystals can be very small initially and grow through a process of agglomeration and recrystallization.
The structure of TiAl.sub.3 present is dependent solely on the process used. It does not depend on composition. It is possible to get 100% blocky or 100% needle structures, or any mixture in between.
The structure of aluminum-titanium grain refiners containing boron have historically been an extension of what has been said above for boron-free (Al-Ti) grain refiners. For any given composition, the resulting structure has been a mixture of TiAl.sub.3 and TiB.sub.2 crystals in a matrix of aluminum saturated with titanium and boron. The prior art has considered the borides to exist only as discreet particles (usually having a hexagonal plate morphology), and the morphology of the TiAl.sub.3 crystals has been either blocky or needle-like. In other words, the TiAl.sub.3 morphology in the ternary (Al+Ti+B) follows the same rules as in the binary (Al-Ti) grain refiners. The only apparent difference is the presence of "free" (Al.Ti)B.sub.2 or TiB.sub.2 crystals.
Examples of attempts to control the blocky and needle structures may be found typically in U.S. Pat. Nos. 3,785,807; 3,857,705; and 3,961,995. These patents disclose several concepts to obtain improved grain refining alloys. These disclosures are often contradictory and do not clearly solve the problems.